Enigmail

Enigmail can encrypt/sign mail when sending, and also decrypt/authenticate received mail. It can also import/export public keys. Enigmail supports both the inline PGP format, and the PGP/MIME format (RFC 3156), which can be used to encrypt attachments.

Enigmail is cross-platform like Mozilla, although binaries are supplied only for selected platforms on this website. Enigmail uses inter-process communication to execute command-line versions of GPG or PGP to carry out encryption/authentication.